1. Field of the Invention
The present invention relates to a distance measuring device for measuring a distance from a phase difference of at least two periodic signals by using an optical frequency comb. Specifically, the present invention relates to a distance measuring device for measuring a distance even when fluctuations occur in the frequency of an oscillator. In this case, the oscillator is used for beating down the frequency of a periodic signal.
2. Description of the Related Art
A distance measuring device using a femtosecond mode-locked pulse laser is known (for example, Japanese Unexamined Patent Application Publication No. 2006-184181). The mode-locked pulse laser has frequency spectra that line up at regular intervals in a cyclic frequency of optical pulses and that are made of multiple discrete spectra (in a longitudinal mode) in which the phases between modes are aligned. Since multiple longitudinal modes at precise intervals line up in the form of teeth of a comb, the optical pulses are called an “optical frequency comb”. In a distance measuring device using the optical frequency comb, a laser is irradiated on an object, and the light reflected by the object is received by a light receiving element. Then, a frequency component having high resolution for measuring a distance is selected from frequency components in large spectrum width, and this selected frequency component is used as a measuring tool. The optical frequency comb is referred to in the article “A New Optical Measuring Tool”, http://www.aist.go.jp/aist_j/museum/keisoku/komu/komu.html.
The distance measuring device disclosed in Japanese Unexamined Patent Application Publication No. 2006-184181 is shown in FIG. 9. The distance measuring device includes a laser unit 1, a dividing device 2, a reference light receiving unit 3, and a measuring light receiving unit 6. The dividing device 2 divides a laser light from the laser unit 1 into reference light and distance measuring light. The reference light receiving unit 3 receives the reference light and outputs multiple beat signals. The measuring light receiving unit 6 receives the distance measuring light and outputs multiple beat signals. The distance measuring device further includes a first filter 14, a second filter 11, an oscillator 30, a first mixer 31, a second mixer 32, and a phase difference measuring circuit 12. The first filter 14 extracts components including a frequency component for measuring a distance, from a received light signal in the measuring light receiving unit 6. The second filter 11 extracts components including a frequency component for reference, from a received light signal of the reference light receiving unit 3. The frequency components from the first filter 14 and the second filter 11 (for example, around 40 GHz) are multiplied by a frequency (40 GHz+10 MHz) at the first mixer 31 and the second mixer 32. This frequency (40 GHz+10 MHz) is slightly different from the frequency components from the first filter 14 and the second filter 11 and is oscillated by the oscillator 30. As a result, the frequency components from the first filter 14 and the second filter 11 are beaten down to low frequency components (10 MHz) (local oscillator method). The low frequency components are input to the phase difference measuring circuit 12 as a distance measuring signal and a reference signal.
In this case, there may be a case in which the oscillator 30 generates a frequency having time fluctuations. If the time fluctuations occur, fluctuations are also generated in the frequencies of the distance measuring signal and the reference signal that are beaten down.
For example, when a frequency of the oscillator 30 has fluctuations of ±100 ppm, the variation of the frequency is ±4 MHz. Therefore, frequencies of processing signals that are beaten down by the first mixer 31 and the second mixer 32 vary from 6 to 14 MHz. Thus, even when the time fluctuations of the oscillator 30 are reduced to approximately ±10 ppm with respect to the frequency (40 GHz) extracted from the received light signal, large fluctuations are generated in a signal that is beaten down. In this case, the variation of the frequency of the signal is ±400 kHz with respect to 10 MHz. Therefore, the frequency of the processing signal varies, and extraction of frequency components by a filter becomes difficult, whereby it is difficult to measure a phase difference between the processing signals. Accordingly, an oscillator having high accuracy, which generates a frequency having greatly decreased fluctuations, is required.